WO1998017478A1

WO1998017478A1 - Continuous ink jet printer pump control

Info

Publication number: WO1998017478A1
Application number: PCT/GB1997/002852
Authority: WO
Inventors: James P. Mckee
Original assignee: Domino Printing Sciences PLC
Current assignee: Domino Printing Sciences PLC
Priority date: 1996-10-23
Filing date: 1997-10-16
Publication date: 1998-04-30
Anticipated expiration: 1999-04-23

Abstract

A continous ink jet printer has an ink reservoir (R) and a print head (P). An ink circuit connects the reservoir with the print head and incorporates two pumps (SP, RP), the first, an ink supply pump (SP), supplying ink from the reservoir to the print head and the second, a return pump (RP) returning unused ink from the print head to the reservoir. A single variable speed motor (M) drives both pumps. An orifice or restriction (FC1) is connected on one side to the reservoir (R) and on the other side to the ink supply line (SL) between the supply pump and the print head, so that ink is bled back to the reservoir from the supply through the orifice or restriction (FC1) during operation of the printer.

Description

CONTINUOUS INK JET PRINTER PUMP CONTROL

The present invention relates to continuous ink jet (CIJ) printers and, more particularly, to an apparatus for controlling ink flow within such printers.

In using a CIJ printer ink is pumped continuously through one or more nozzles, the stream of ink passing through the or each nozzle being broken up into individual droplets under the action of a piezoelectric or similar oscillator and the droplets being selectively charged and deflected in an electric field for printing purposes, non- printable droplets passing into a gutter from which the ink is returned to a reservoir feeding the pump.

As the flow rates of ink to the print head containing the nozzle or nozzles and, in return, from the gutter at the print head, are different, separate pumps may be used. However, in seeking to simplify printers of this type it has previously been proposed to drive the pumps from a single motor. The motor drives the pumps therefore together and each of the pumps is optimised for its particular use. A pressure regulator may be used to adjust the pressure of ink supplied to the print head or pump speed may be controlled. Typically, the pumps are gear pumps, with different pump gears being used for the supply and gutter-return flows.

The flow rates of the supply and return will vary from application to application.

The flow of ink required at the print head is, for a given application, effectively, constant, but the return flow through the gutter may be variable or may require greater air flow. Thus, on the one hand, for a given application, the motor requires to be driven at a substantially constant speed to provide a uniform supply of ink to the print head whereas, on the other hand, there is, preferably, a requirement for variable speed depending upon operating variables such as print head elevation, ink supply conduit length, gutter return tube diameter, ink viscosity, ink surface tension, number of nozzles (in the case where a different print head containing a different number of nozzles may be installed or perhaps a different nozzle may be installed in the same print head) , etc.. These conditions appear to be mutually exclusive.

In order to overcome these problems the motor can be operated at a speed which corresponds with the maximum required gutter flow rate for the printer. This, of course, is wasteful and may result in the entrainment of considerable volumes of air within the ink returned from the gutter, in turn causing additional problems such as high ink solvent consumption, increased power requirements, etc.

According to the present invention, a continuous ink jet printer incorporating two pumps, one for ink supply and one for return vacuum, driven by a single variable speed motor, and a reservoir from which ink is drawn by the supply pump and to which ink is returned by the return pump; includes an orifice or restriction connected between the reservoir and the ink supply downstream of the supply pump.

The orifice or restriction, which may be fixed or variable in size, is used to bleed ink flow from the supply pump side back to the reservoir. Since the print head flow requirement is substantially constant, establishing the bleed flow rate with the orifice or restriction permits speed adjustment to enable adjustment of the return pump flow for controlling gutter flow, without compromising the required supply flow to the print head. The dual-circuit pump motor will normally be driven to the speed necessary to provide the required flow of ink to the print head nozzle or nozzles, and the orifice or restriction can be adjusted in size or else, if it is of fixed size, it can be selected based on the orifice or restriction size which will provide the required gutter flow. If the orifice or restriction is variable in size and the effective flow area is increased, or if a larger flow fixed orifice or restriction is installed, the increased size of the orifice or restriction provides a greater by- pass flow to the reservoir resulting in an increase in motor speed in order to maintain the print head flow, thereby resulting in an increase in gutter flow through the return pump. Conversely, if the orifice or restriction size is reduced or a smaller orifice or restriction installed, reduced by-pass flow results, in turn causing the motor to decrease in speed to maintain the print head flow and resulting in a decrease in the return flow from the gutter.

If the orifice or restriction is of fixed size then it may be opened or closed, for example by means of a valve such as a solenoid valve, in order to achieve similar changes in speed of the pump motor in order to maintain the supply flow to the print head whilst changing the return flow from the gutter. The orifice or restriction may be automatically adjusted using a rotary, spool, or similar valve or a rotary actuator on a needle or metering valve providing a variable restriction, to adjust the pump speed/gutter flow rate based on a gutter flow parameter (eg vacuum) , adjustment taking place while ink is being pumped to the print head to generate flow to the gutter, but during periods of non-printing.

Using a single motor for both pumps results in an ink system cost reduction over the use of independent pumps and drive motors and the invention provides the flexibility required to achieve variable gutter flow at the same time enabling a dual pump/motor combination to be used for a number of different ink jet systems having differing flow requirements. Conventionally, pump manufacturers provide gears of stock sizes and customised moulded gears are prohibitively expensive. The invention allows the same pump to be used in different systems and adjusted/controlled in order to suit the needs of differing inks and flow conditions.

Examples of the invention will now be described with reference to the accompanying drawings in which: Figure 1 is a diagram of an ink circuit for a multi- nozzle CIJ printer;

Figure 2 is a diagram of an alternative ink circuit for a multi-nozzle CIJ printer, and;

Figure 3 is a diagram of another alternative ink circuit for a multi-nozzle CIJ printer.

Referring to Figure 1, an ink circuit has an ink reservoir R from which ink is pumped by means of a supply pump SP and to which ink is returned by a return vacuum pump RP, both of which are driven by the same variable speed motor M via a common drive shaft. The supply pump SP supplies ink to the print head P through a supply line SL via filters Fl and F5, and a solenoid valve SV8 and filter F6 located in the print head P. Diagrammatically illustrated in the print head P is an ink chamber C from which plural individual streams of ink IS are emitted in use through a corresponding plurality of nozzles (not shown) . In use, when printing, the ink streams are broken up into droplets by an ultrasonic vibrator (not shown) and droplets which are not required to be printed are directed into a gutter G from which ink is returned through a return line RL via a solenoid valve SV3 and filters F2 and F4 under the action of the vacuum return pump RP. Solenoid operated valves SV1 and SV2 allow ink and solvent make-up respectively into the circuit as required during operation. This is conventional and, as it does not relate to the inventive aspect of the circuit, will not be further described. Solvent can also be fed to the print head through solenoid valve SV7 for flushing at shut down of the printer. The pressure of ink is monitored, in this example, at the print head P by a transducer T6 (or, alternatively ink jet velocity is monitored) and information from the transducer is fed back to the variable speed motor M in order to achieve a substantially fixed pressure supply of ink or fixed jet velocity to the print head P. The required pressure will depend upon the ink being used which in turn will vary from application to application and, similarly, the amount of ink returned depends upon the application (eg. different nozzles and other parameters mentioned above) and therefore the ratios of the flows produced by the supply pump SP and the vacuum return pump RP will vary from application to application. However, the gears available for the pumps SP, RP may not match the desired flow rates exactly and therefore a flow control restriction FCl is provided between the supply line SL and the reservoir R in a by-pass line BL, the flow control restriction FCl being sized appropriately or adjustable in order to produce the desired flow ratio between the supply pump SP and the vacuum return pump RP for the given application.

In the present example the flow control restriction FCl is shown as fixed in size and is preferably readily changeable in order to enable the printer to operate under varying flow rate conditions, for example for use with different inks or different nozzles. Alternatively, the flow control restriction FCl may be variable and capable of adjustment to accommodate different flow rates which may be required.

On the by-pass line BL there is a temperature sensor T10 for measuring the temperature of the ink and, in parallel with the by-pass line BL and controlled through a solenoid valve SV13 , there is an ink sampling line ISL having a viscometer VM1 for measuring the viscosity of the ink. A pressure release valve PRV1 is also provided to allow flow through the by-pass line BL to the reservoir R. Measurements of viscosity enable control of the addition of solvent to the ink, via the solenoid operated valve SV2 , to maintain the desired viscosity. Control lines Cl and C2 are connected to the supply line SL via solenoid valves SV12 and SV5 respectively and to the by-pass line BL via solenoid valves SV11 and SV4 respectively to control ink actuated movement of the gutter G and charge electrodes (not shown) in the print head.

A further flow control orifice FC2 is provided in the print head bleed line PBL for limiting the return flow of ink from the print head during bleeding of air from the print head when the printer is shut down. The remaining components of the ink circuit shown in Figure 1, whilst being necessary for operation of the printer, form no part of the present invention and will therefore not be further described.

In an alternative circuit shown in figure 2 the return line RL is connected to the print head bleed line PBL via solenoid valve sv2 , sv7 & SV10. This allows flushing of the print head P with solvent or make-up fluid during shutdown in the manner described in our British Patent Application no. 9719705.7. In that application we describe how solvent can be drawn into the bleed line PBL via solenoid valve SV7 , with the valve SV9 closed and valve SV10 open to connect the bleed line PBL to the return pump RP, and then, with valves SV7 & SV10 closed, forced through the print head P under the action of pressure in the bleed line PBL resulting from opening of the solenoid valve SV9 to connect the bleed line PBL to the supply pump SP.

The circuit shown in figure 3 is similar to that shown in figure 2, but an additional supply pump SP ' and return pump RP' are provided, along with motor M¹. The supply pump SP • is connected to the reservoir R and via an additional supply line SL' to the supply line SL. Similarly, the return pump RP ' is connected to the reservoir R and via the return line RL' to the gutter G. Thus, the pumps SP ' and RP ' operate in parallel in the ink circuit to the supply pump SP and return pump RP respectively. This arrangement increases the overall pumping power of the system, enabling the use of larger print heads.

The circuit shown in figure 2 further includes a reservoir bleed line RBL between the reservoir R and the bleed line BL. Similarly, the two pump circuit of figure 3 has two reservoir bleed lines RBL and RBL' . Along the reservoir bleed lines RBL and RBL' there are provided flow control orifices FC3 and FC4 respectively. The orifice FC3 (and FC4) allows control of the ink flow from the reservoir R to the return line RL (and RL') so that the return pump R (and R') can be "wetted" on start up.

Claims

1. A continuous ink jet printer having an ink reservoir; a print head; and an ink circuit incorporating two pumps, an ink supply pump for supplying ink from the reservoir to the print head and a return pump for returning unused ink from the print head to the reservoir, and a single variable speed motor driving both pumps; characterised by an orifice or restriction connected on one side to the reservoir and on the other side to the ink supply between the supply pump and the print head, whereby ink is bled back to the reservoir from the supply through the orifice or restriction.

2. The continuous ink jet printer of claim 1, wherein the orifice or restriction is fixed in size.

3. The continuous ink jet printer of claim 2, further including a valve for opening and shutting the orifice or restriction.

4. The continuous ink jet printer of claim 1, wherein the orifice or restriction is variable in size.

5. The continuous ink jet printer of claim 4, further including means for automatically adjusting the size of the orifice or restriction.